It is well-known to manufacture xylenes by the alkylation of toluene and/or benzene with methanol, and in particular to selectively make paraxylene (PX) product using zeolite catalyst. See, for instance, U.S. Pat. Nos. 4,002,698; 4,356,338; 4,423,266; 5,675,047; 5,804,690; 5,939,597; 6,028,238; 6,046,372; 6,048,816; 6,156,949; 6,423,879; 6,504,072; 6,506,954; 6,538,167; and 6,642,426. The terms “paraxylene selectivity”, “para-selective”, and the like, means that paraxylene is produced in amounts greater than is present in a mixture of xylene isomers at thermodynamic equilibrium, which at ordinary processing temperatures is about 24 mol %. Paraxylene selectivity is highly sought after because of the economic importance of paraxylene relative to meta- and orthoxylene. Although each of the xylene isomers have important and well-known end uses, paraxylene is currently the most economically valuable, serving as an intermediate in such important and diverse end uses as bottle plastic and polyester fibers.
In the process, typically toluene and/or benzene are alkylated with methanol, in the presence of a suitable catalyst, to form xylenes in a reactor in a system illustrated schematically in FIG. 1, wherein a feed comprising reactants enter fluid bed reactor 11 via conduit 1 and effluent comprising product exits through conduit 5, and the catalyst circulates between fluid bed reactor 11, apparatus 12, which strips fluid from the catalyst, and catalyst regenerator 13, via conduits 2, 3, and 4, respectively. Water is typically co-fed with toluene and methanol to minimize toluene coking in the feed lines and methanol self-decomposition. Other side reactions include the formation of light olefins, light parafins, as reactions that convert paraxylenes to other xylene isomers or heavier aromatics.
Apparatus suitable for each of the elements shown in FIG. 1 are per se well-known in the art, however as one of ordinary skill in the art will recognize simply picking and choosing for what is available will not arrive at a commercially viable system. There are numerous problems to overcome, and arriving at a system which can compete with other known methods of making paraxylene selectively, such as transalkylation and toluene disproportionation, is the subject of intense current research in the industry. A few of the problems are outlined below.
Gas bubbles formed at the bottom of a fluid bed will grow as they rise through the bed until they reach a maximum stable bubble size. Because the bubbles will grow at different rates, there will be typically a broad distribution of bubble sizes in a fluidized bed. A broad bubble size distribution can cause significant gas phase back-mixing both at a local level due to formation of turbulent eddies as well as at the global level due to uneven axial velocity profiles across the horizontal direction. Such gas back-mixing keeps portions of the desired product in contact with active catalyst for longer than the expected plug flow reactor residence time. On the other hand, while back-mixing increases gas phase residence time, another phenomenon can simultaneously result in decrease of the gas phase residence time. Large bubbles can also form continuous gulf-streaming flow structures which result in gas by-pass, which allows gas to quickly pass through the bed with little contact with catalyst.
Poor contact between gas and active catalyst in the by-pass zone results in reduced reactant conversion and low fluid bed (reactor volume) utilization.
The use of a baffled system is known from U.S. Pat. No. 7,935,857. Other relevant references include U.S. Pat. Nos. 4,251,484; 3,982,903; 4,855,111; 6,642,426; and GB 803458.
While backmixing is generally considered good for methanol conversion, the present inventors have discovered that staged injection in the presence of a baffle system avoids secondary reactions such as isomerization of the desired paraxylene to its less desirable isomers, and improves selectivity to paraxylene from about 60-70 wt % to above 80 wt %, such as to 80-90 wt % in embodiments. Without wishing to be bound by theory, the present inventors have surprisingly discovered a process and apparatus or system adapted therefore, wherein the combination of reduced gas phase back-mixing and by-pass phenomena can work in concert to improve both conversion and selectivity, as well as increase catalyst utilization, by the use of staging baffles in a deep fluid bed to yield smaller and more uniform, and thus controllable, bubble sizes, and combining staged methanol injection to the reactor bed with the use of structured packing layers as staging baffles selectivity to more efficiently provide the desired products.